Frequency domain digital beam forming operates on the samples baseband complex envelope of the beam signal. In conventional digital beam forming architecture, a beam in the transmit direction is generated by directing a copy of the signal sample sequence, multiplied by an element specific complex weight, to each antenna array element. To detect a beam in the received direction the baseband complex envelope samples on each array element are multiplied by element specific complex weights and the products summed on a sample by sample basis to generate the desired beam signal. With an antenna array of N-elements agile digital beam forming thus requires N-complex-complex multiplications per beam sample.
In a known variation of such conventional architecture, the set of orthogonal beams defined by the antenna array geometry is generated simultaneously by Discrete Fourier Transform (DFT) across the array element samples. The DFT is implemented using an appropriate Fast Fourier Transform (FFT. This reduces the number of multiplications per beam sample to the order of log.sub.2 N.
Such conventional techniques for frequency domain digital beam forming are described in "Multi Dimensional Digital Signal Processing" by Dan E. Dudgeon and Russel M Mersereau, published by Prentice-Hall 1984.
In applications where the orthogonal beams generated by FFT beam forming are too widely spaced to give the desired density of beams over the coverage area, additional, non-orthogonal, beams may be interpolated between the orthogonal beams by extending the transform size beyond that defined by the physical array elements. This means zero extending the array in the receive direction and windowing the extended transform output in the transmit direction. However the increase in transform size, allied to the fact that only a subset of the beams thus generated are over the coverage area, severely compromises the computational efficiency of generating the beams this way, to the extend that there may be little or no computational advantage in using FFT to generate agile beams in this way.
There is thus a need to provide a generally improved digital signal processing mehtod and apparatus for beam forming using an N-element phased array antenna which substantially retains the computational efficiency of FFT beam forming to generate the N orthogonal beams and at the same time provide the ability to generate additional, fully steerable beams for significantly lower computational cost than would be required by either of the two conventional techniques hereinbefore described.